In order to develop a transfection system with human cells in which the effect of cellular DNA from human tumors and previously cloned onc genes can be assayed, we have screened a number of non-tumorigenic human cell lines for several characteristics suiting them for use as recipients in such a transfection system. The SV80 cell line and a hybrid between HeLa and a normal diploid fibroblast can both be transfected for cloned selectable markers with a high efficiency. They have a cryptic tumorigenic potential, that is, under certain rare circumstances, they will form tumors in nude mice. SV80 cells were transfected with mixtures of a selectable marker, guanine phosphoribosyl transferase (gpt) and cloned onc genes. Introduction of MSV or EJ sequences by cotransfection with the selectable marker did not render the cells tumorigenic nor result in the obvious formation of foci, although individual cells showing expression of the MSV p30 antigen showed morphological alterations and virus could be rescued from the MSV-transfected cells. In a preliminary experiment, transfection using a mixture of MSV DNA and adenoviral E1a and E1b DNA, did result in tumor formation. The HeLa/fibroblast hybrid line could be cotransfected at low efficiency in comparison with rodent cell lines. Cells cotransfected with EJ DNA did not become tumorigenic nor did they show any morphological alteration. Messenger RNA studies to date indicate that the non-tumorigenic hybrids do not show suppression of H-ras or myc expression, when compared to tumorigenic derivatives isolated from them by others. We have cloned a 5' EcoRI fragment of an N-ras gene from a human gastric adenocarcinoma. This N-ras fragment is active in transfection assays on NIH 3T3 cells when ligated to the 3' EcoRI fragment from the normal human gene.